1. Field of the Invention
The present invention relates to a lithographic apparatus having a gas flushing device and relates to a gas flushing device for flushing a substantially laminar flow of gas across a predetermined space in, for example, a lithographic apparatus.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (including, for example, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
More particularly, the invention relates to a lithographic apparatus that includes: an illumination system for providing a projection beam of radiation, and a support structure for supporting a patterning device. The patterning device serves to impart the projection beam with a pattern in its cross-section. The apparatus also includes a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate.
The projection system usually includes an optical component such as a lens for focusing the beam onto the target portion of the substrate. It is, however, as will be pointed out in slightly more detail further on in this specification, also possible to direct the beam via reflective components such as mirrors. The surfaces of these optical components may interact with gaseous contaminants in such a way that the transmission of light or the reflection of light occurs in a less accurate manner than would be the case had the surfaces of the optical components not interacted with these gaseous contaminants.
The result of the interaction may result in a hindrance of the transmission or reflection of light via these optical components. That is, due to the interaction, a layer may be formed on or into the surface of the optical opponent, changing the transmission coefficient or reflection coefficient of the optical component. This change may have a permanent nature. As the optical components are usually very expensive components of the apparatus, a reduction of the lifetime of these components due to interaction with these particles is highly undesirable. In this document often the term “particles” is used as part of contaminating gases. It should be understood that these particles may have molecular dimensions.
The gaseous contaminants interacting with these optical components may be released by the wafer substrate, for example, before exposure of the substrate to the beam as a result of, for example, outgassing, during exposure of the substrate to the beam as a result of the removal of material from the substrate, or after exposure of the substrate to the beam as a result of for example baking out the substrate. These gaseous contaminants may also be present in the lithographic apparatus. Particles which may interact with the surface of an optical component may also be formed during for example production of UV radiation. The interaction with the optical components may occur under the influence of the radiation. Most often though, gases from the wafer or from elsewhere present in the lithographic apparatus interact under the influence of the radiation with a coating on the surface of an optical component. Due to a chemical reaction, crystals are formed, which negatively affect the performance of the optical component. To remove these crystals, the apparatus has to be opened up, which results in down time and hence expenses. Sometimes the crystals cannot be cleaned away and the optical element has to be replaced by a new one.
One way of preventing the interaction of the gaseous contaminants with a surface of an optical component is carried out by flushing a flow of gas along a surface of an optical component to drag away, within the flow of gas, contaminating particles which were about to approach the surface of the optical component. It may also be useful to provide the flow of gas across the patterned projection beam in order to prevent particles traveling along a path followed by the patterned projection beam from reaching a surface of the optical components. A flow of gas may also be provided both across the patterned projection beam and along a surface of an optical component. If the flow of gas is provided across the patterned projection beam, or any other beam of radiation for that matter, the flushing gas is preferred to be substantially non-absorbent of the radiation used in the projection system. The flow of gas is also strongly preferred to be a substantially laminar flow of gas in order to improve the shielding effect of the flow of gas.
EP 1098226 A2 describes a lithographic apparatus with a gas flushing device. The device includes a number of spaces, stacked in a parallel fashion on top of each other. In use, gas flows along the parallel direction through each of these spaces. To ensure laminar flow and minimize turbulence, the various spaces are spatially separated from one another. Although this system provides, in many situations, a useful way of flushing a laminar flow of gas across the patterned projection beam and/or along a surface of an optical component, the system disadvantageously occupies too much space, as determined by the minimum distance between, for example, a surface of an optical component and a source for particles. In fact, due to the presence of a gas flushing device as described in this prior art, the distance between a bottom of the projection system and the substrate table has to be undesirably large.
In this context, it is worth noting that in the field of projecting a beam onto a target portion of the substrate, the current tendency is aimed at more accurate projections, demanding a higher numerical aperture number NA. Between the substrate and the bottom of, for example, a final lens element of the projection system, there needs to be a distance for allowing flushing away the gaseous contaminants. If the distance needs to be large and the NA number needs to be high, the bottom of the lens has to be relatively large.
As fabrication, machining etc. of the bottom of this lens is a very expensive process, it is preferred that the bottom of the lens be much smaller. With the high NA number, the distance between the substrate and the bottom of the lens is preferably as small as possible; and is large enough to just be able to transport the wafer without hitting the lens.